Application of high strength steel sheet to automobile bodies has recently been progressing rapidly due to demands for crash safety and weight reduction. Employing high strength steel sheet in automobile bodies enables the energy absorbed in a crash to be increased and buckling strength to be increased, without increasing sheet thickness. However, there is a concern that the steel sheet may fracture during press forming or crash deformation, due to the ductility and bendability of steel sheet decreasing along with increasing strength of the steel sheet. In order to determine the state of the steel sheet during press forming and crash deformation, there is an increasing need to determine steel sheet fracture with high precision using finite element methods.
The use of sheet thickness reduction ratios or forming limit diagrams (FLDs) is known for evaluating the formability and the amount of leeway with respect to fracture during crash performance evaluation.
FIG. 1 illustrates an example of a FLD in a strain space from uniaxial deformation to plane strain deformation, and from plane strain deformation uniform biaxial deformation.
As illustrated in this drawing, the FLD is a diagram illustrating a relationship between the major strain and minor strain that give a fracture limit. Brief explanation follows regarding experimental measurement methods for FLDs. First, a metal sheet, having a circular or lattice pattern drawn on the surface thereof, by etching or the like, is fractured by hydraulic forming or stretch forming with a rigid tool. Next, the fracture limit strain is determined from the amount of deformation in the pattern on the surface of the fractured metal sheet. Load is applied to the metal sheet so as to change an in-plane strain ratio, and a fracture limit curve is obtained by plotting the fracture limit strain for each strain ratio.
Combined application of Hill's criterion and Swift's one, the Marciniak-Kuczynski model, the Storen-Rice model, and the like are known, as theoretically predicting FLDs. In ductile fracture of materials, deformation caused by localized necking is generated at localized positions. The criterion is often considered identical to the localized necking generation limit due to the generation of localized necking causing materials to fracture within a very short time. Therefore, the predicted criterion is often treated as a plastic instability phenomenon. In the conventional fracture prediction method described in Japanese Patent Application Laid-Open (JP-A) No. 2012-33039, risk of fracture is predicted by comparing positional relationships between a fracture limit curve and a strain state for each element obtained from simulation by a finite element method. Namely, it is determined that there is a fracture or that there is a high risk of fracture when strain in a deformation process obtained from simulation reaches a limiting strain defined by the fracture limit curve.